AD7980BCPZ-R2 datasheet(19/28 Pages) AD

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Data Sheet

AD7980

Rev. F | Page 17 of 26

VOLTAGE REFERENCE INPUT

The AD7980 voltage reference input, REF, has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins, as explained in the Layout section.

When REF is driven by a very low impedance source, for example,

a reference buffer using the AD8031 or the ADA4805-1, a ceramic

chip capacitor is appropriate for optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR435 reference.

If desired, a reference-decoupling capacitor value as small as

2.2 μF can be used with a minimal impact on performance,

especially DNL.

Regardless, there is no need for an additional lower value ceramic

decoupling capacitor (for example, 100 nF) between the REF

and GND pins.

POWER SUPPLY

The AD7980 uses two power supply pins: a core supply, VDD, and

a digital input/output interface supply, VIO. VIO allows direct

interface with any logic between 1.8 V and 5.0 V. To reduce the

number of supplies needed, VIO and VDD can be tied together.

The AD7980 is independent of power supply sequencing between

VIO and VDD. Additionally, it is very insensitive to power supply

variations over a wide frequency range, as shown in Figure 29.

Figure 29. PSRR vs. Frequency

The AD7980 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate. This makes the device ideal for low sampling

rate (even of a few Hz) and low battery-powered applications.

Figure 30. Operating Currents vs. Sampling Rate

DIGITAL INTERFACE

Though the AD7980 has a reduced number of pins, it offers

flexibility in its serial interface modes.

The AD7980, when in CS mode, is compatible with SPI, QSPI™,

and digital hosts. This interface can use either a 3-wire or 4-wire

interface. A 3-wire interface using the CNV, SCK, and SDO signals

minimizes wiring connections useful, for instance, in isolated

applications. A 4-wire interface using the SDI, CNV, SCK, and

SDO signals allows CNV, which initiates the conversions, to be

independent of the readback timing (SDI). This is useful in low

jitter sampling or simultaneous sampling applications.

The AD7980, when in chain mode, provides a daisy-chain

feature using the SDI input for cascading multiple ADCs on a

single data line similar to a shift register.

The mode in which the device operates depends on the SDI level

when the CNV rising edge occurs. The CS mode is selected if

SDI is high, and the chain mode is selected if SDI is low. The

SDI hold time is such that when SDI and CNV are connected

together, the chain mode is selected.

In either mode, the AD7980 offers the flexibility to optionally

force a start bit in front of the data bits. This start bit can be

used as a busy signal indicator to interrupt the digital host and

trigger the data reading. Otherwise, without a busy indicator,

the user must time out the maximum conversion time prior to

readback.

The busy indicator feature is enabled in the CS mode if CNV or

SDI is low when the ADC conversion ends (see Figure 34 and

Figure 38). The busy indicator feature is enabled in the chain

mode if SCK is high during the CNV rising edge (see Figure 42).

1000

FREQUENCY (kHz)

100

10.000

1.000

0.100

0.010

0.001

100000

SAMPLING RATE (SPS)

10000

1000000

IVDD

IVIO

IREF

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